tox: fix docs generation
Docs generation doesn't work because of some dependency issue, downgrading mistune fixes it.
This time I've tested it with r flag which redownloads all
dependencies (tox -r -e docs) so it should be ok.
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Docs generation doesn't work because of some dependency issue, downgrading mistune fixes it.
This time I've tested it with r flag which redownloads all
dependencies (tox -r -e docs) so it should be ok.